1. Field of the Invention
The invention relates to an arrangement for producing information identifying non-occupied transmission paths in a digital transmission system. The transmission system is composed of a currently operating system for transmitting useful information in accordance with the time-division multiplex method, and a stand-by system.
2. Prior Art
The German "Auslegeschrift" DE-AS 27 53 420 discloses an arrangement for switching from operational to stand-by transmission paths in transmission systems for digital signals. In this known arrangement, predetermined bit positions in the time-division multiplex frame are used for identifying non-occupied transmission paths. However, these predetermined bit positions in the time-division multiplex frame signal are not available in every time-division multiplex transmission system. In the frame structure of a standardized 2 Mbit/s time-division multiplex systems, the frame alarm word and the system drawn word are alternatively transmitted in the first time slot of the frame. In the system alarm word, the bit positions 5 to 8 are freely available for the system user for transmitting additional information. Up to 16 different messages can be transmitted by means of these four bits that are commonly referred to as Y-message bits. In a standardized 8 Mbit/s and also in a standardized 34 Mbit/s time-division multiplex transmission system, such Y-message bits are not available. In a standardized 140 Mbit/s time-division multiplex transmission system, the frame structure comprises two Y-message bits in the bit positions 15 and 16 in set I (cf. for example TEKADE Technische Mitteilungen 1984, pages 17 to 22, more specifically page 17).
If the Y-message bits of the time-multiplex frame are used for the purpose of identifying non-occupied transmission paths, then this type of identification cannot be utilized for all types of time-division multiplex transmission systems, as such Y-message bits are not available in every frame structure (compare those of the 8 Mbit/s and 34 Mbit/s time-division multiplex transmission systems) or only a small number of different messages can be transmitted (in the 140 Mbit/s time-division multiplex transmission system having two Y-message bits, only four different messages can be transmitted).
In the arrangement disclosed in German "Auslegeschrift" DE-AS No. 27 53 420, the substitute switching from n currently operating systems to one stand-by system is effected by means of control arrangements included in the line terminal equipment of the stand-by system. The control arrangements always insert in the position of the Y-message bits an operational status character or, in the case of mal-functioning, the addresses of the currently operating system to be switched-over; the operating status character in the two transmission directions of the stand-by system can also be a standardized alarm-indication signal, commonly referred to as AIS.
The AIS is used in the digital transmission technique to report a transmission disturbance, for example a frame alignment disturbance (cf. the European Patent EP-Bl-0 031 943), and is a permanent-1 signal. According to European Patent, EP-Bl-0031943, this signal is produced by an AIS-generator arranged in the line terminal equipment and is used to suppress a subsequent alarm in the subsequent signal path. This prevents alarm signals from occurring in the entire down-stream section of the transmission path in the event of a disturbance in the upstream sections of the transmission path. From European Patent EP-Bl-0 031 943 it is known that it is also possible to include such AIS-generators in the intermediate regenerators (repeaters). In addition to the AIS generators, the line terminal equipment also includes supervision arrangements connected to a central monitoring station via a control line. The AIS-generator in question can be connected to the output of the line terminal equipment at the receiving side by means of a change-over switch that is remotely controlled by the central station. Furthermore, it is known from the above-mentioned publication (TEKADE Technische Mitteilungen 1984, pages 17 to 22) to provide in a plesiochronous digital signal multiplexer a supervision circuit at the interface. E 140 Mbit/s (SSTEE) for recognizing the AIS-signal in the sequence digital signal. On page 20 of said publication, it is set forth that for stand-by switching purposes a switch-over from the operational input B at the input of the SSTEE-interface to a second (stand-by) input E can be effected, which change-over action of the switch can be effected manually or automatically and be remotely controlled. Before the change-over action, no signal is transmitted through the stand-by channel in the 140-Mbit/s plane.